The present invention is directed to nitrogen-containing heterocyclic epoxy curing agents, compositions, and methods.
Epoxies are known for their excellent adhesion, chemical and heat resistance. In addition they also have good-to-excellent mechanical properties, and good electrical insulating properties. Cured epoxy resin systems are found in an extensive range of applications within the coatings, adhesives, and composites markets. Specific examples include epoxy composite using carbon fiber and fiberglass reinforcements, structural adhesives, protective coatings for metal surface, and construction products for concrete, cementitious or ceramic substrates, often referred to as civil engineering applications such as formulations for concrete flooring.
Cured epoxy resin systems consist of two components that can chemically react with each other to form a cured epoxy, which is a hard, duroplastic material. The first component is an epoxy resin and the second component is a curing agent, often referred to a hardener. Epoxy resins are substances or mixtures which contain epoxide groups. The curing agents include compounds which are reactive to the epoxide groups of the epoxy resins, such as amines, carboxylic acid, and mercaptanes (H. Lee and K. Neville “Handbook of Epoxy Resins” McGraw Hill, New York, 1967, pages 5-1 to 5-24). The epoxy resins can be crosslinked or cured by curing agents. The curing process is the chemical reaction of the epoxide groups in the epoxy resins and the reactive groups in the curing agents. The curing converts the epoxy resins, which have a relatively low molecular weight, into relatively high molecular weight materials by chemical addition of the curing agents to the epoxy resins. Additionally, the curing agent can contribute to many of the properties of the cured epoxy.
As noted above, epoxy resins are crosslinked or cured in order to develop certain characteristics. Many of these industrial applications mentioned require technology which can provide a faster return to service under application conditions specific to the industries in which they are used. For coatings and civil engineering, this is mainly the market need for improved reactivity and performance at low application temperatures, typical <10° C. and more often below 0° C., whereas in structural adhesives and composites the requirement can be for high reactivity of the curing agent at elevated temperature in the range 50° C. to 150° C.
However, many epoxy coatings suffer from slow cure at low application temperatures (≤5° C.) and a common side effect of the slow cure is that coatings can develop poor surface defects and a greasy surface appearance referred to as blushing, carbamation of or water spotting. These problems are in part attributed to the slow amine-epoxy reaction rate and partial incompatibility between the amine curing agent and epoxy resin. The incompatibility causes phase separation and amine migration to the coating surface. This results in incomplete amine-epoxy reaction and insufficient crosslinking which can lead to poor physical properties and poor coating performance. If free amine is present on the surface for too long a period, then the amine will react with water and carbon dioxide in the atmosphere and develop a white film on the surface which can lead to poor coating performance. Another problem is that the cure requires longer time for coating to set and dry, which means longer time to return to service or for the subsequent layer to be over coated. Traditionally the industry has used accelerators such as tertiary amines, and phenols and phenolic derivatives such as Mannich base compounds, salicylic acids to speed up amine-epoxy reaction at low temperature. However, incorporation of these species can only be used at low levels since they can cause the epoxy resin to homopolymerize and the resultant system to become brittle. Also they have a significant impact in causing the final epoxy system in be more prone to yellowing.
As also mentioned in the application area of structural adhesives and composites the need to reduce cycle times and increase productivity also requires a high reactivity of the curing agent with the epoxy resin. However, many of the curing agents used today providing high reactivity have drawbacks, which are either technical, such as uncontrolled reactivity, where a severe exotherm can result in damaging component parts during production as well as leading strong yellowing, in addition there are also health and safety concerns, where many materials may exhibit high vapor pressure and/or toxic labeling. Increasing regulatory pressure for both environmental and worker safety creates a need in these markets for curing agents with lower hazard ratings.
U.S. Pat. No. 4,269,742 discloses the preparation and use of Mannich base compounds as epoxy hardener to produce tack-free films at low temperature.
U.S. Pat. No. 6,465,601 discloses Mannich base compounds as accelerators for curable epoxy systems. There is a need in the industry to develop amine-epoxy compositions that are fast cure at low temperature and yellowing resistant.
WO 2013003202 discloses fast curing epoxy resin systems using mixtures of amine hardeners which are based on diethylene triamine (DETA). The epoxy resin systems and hardeners of this application are in particular suited for processes were short cycle times are desirable, such as resin transfer moulding under (high) pressure to make automotive parts. A significant issue with this application is the presence of DETA, which is coming under regulatory pressure because of health and safety aspects. There is a strong desire to replace DETA with an alternative curing agent, while retaining the benefits of good reactivity, such as low initial viscosity, good open time and fast cure. A high glass transition temperature would be a further advantage, if it could be obtained without comprising the needed curing characteristics.
U.S. Pat. No. 2,643,977 discloses a method of inhibiting metal corrosion using reaction product from diethylenetriamine with an aldehyde to form an imidazolidine intermediate and the final product was found to have corrosion-preventing properties.
U.S. Pat. No. 4,877,578 discloses the use of polyamine/formaldehyde reaction products as corrosion inhibitors for refinery overhead systems. The reaction products are described as a complex mixture including alkylene-bridged diethyleneamine triamines.
Chinese Patent Publication CN103333136 discloses a preparation method of a polyaminoamide cationic asphalt emulsifier. The patent discloses a reaction of polyethylene polyamine and formaldehyde, but does not describe the products of the reaction, and only discusses the products of the reaction as an intermediate product which is reacted with an organic acid and a quaternary ammonium reagent to generate a polyaminoamide cationic asphalt emulsifier.
German Patent Publication DE2321509 discloses a method for clarifying colored aqueous waste liquids or waste water, which includes use of water soluble condensation products of an aliphatic amino compound, such as diethylenetriamine, with a C1 to C4 aliphatic aldehyde.
Araki et al. in “Site-Selective Derivatization of Oligoethylenimines Using Five-Membered-Ring Protection Method”, Macromolecules, vol. 21, no. 7, pp. 1996-2001 (1988), disclose a protocol for ring-closing oligoethylenimines to protect selected sites during organic synthesis and derivatization.
Khune and Ghatge in “Amine Aldehyde Condensation Products for Stabilization of Natural Rubber Latex Foam”, Journal of Macromolecular Science: Part A—Chemistry: Pure and Applied Chemistry, col. A(15), no. 1, pp. 153-168 (1981) disclose ring-closing condensation reactions of formaldehyde with various aliphatic amines, but only monoamines or diamines are discussed as starting materials, and not triamines or larger species. These compounds are discussed with respect to their use as stabilizers for natural rubber latex foam.
The disclosure of the foregoing publications including patents and patent applications is hereby incorporated by reference.